Methods of restoration in an ultra-long haul optical network

ABSTRACT

A method includes receiving a restoration indicator associated with a path that includes an optical cross-connect (OXC). The OXC is reconfigured from a standby configuration to a restoration configuration in response to the restoration indicator. An optical signal received in a first direction at a first wavelength is optically regenerated to produce an optical signal in the first direction at a second wavelength. An optical signal received in a second direction at the second wavelength is optically regenerated to produce an optical signal in the second direction at the first wavelength.

BACKGROUND

The invention relates generally to ultra-long-haul optical networks andparticularly to methods associated with restoring a circuit within anultra-long haul optical network.

In some known systems, optical-electrical-optical (OEO) regeneration isused to amplify signals in long-haul links. Such OEO regeneration can beperformed, for example, in cross-connects that define the nodes within anetwork. Restoration within such a network is typically performed atthese cross-connects such that alternative paths can be definedon-the-fly from a pre-established pool of restoration capacity. Becausethe transmitted signals for each link between two adjacent nodes withina path each has a given wavelength, known network restoration techniquestypically need not manage the wavelengths of transmitted signals along arestoration path.

All-optical networks (i.e., networks using optical regeneration withoutelectrical conversion), however, typically need to manage the wavelengthusage for the various links within a restoration path. In addition,unlike networks that use OEO regeneration, networks that use opticalregeneration without electric conversion typically need to managechanges in the cross talk and the total-power-per-link resulting fromthe restoration path.

Thus, a need exists for methods of managing changes in an opticalnetwork to accommodate restoration path actuation and wavelength usagein an efficient and effective manner.

SUMMARY OF THE INVENTION

A method includes receiving a restoration indicator associated with apath that includes an optical cross-connect (OXC). The OXC isreconfigured from a standby configuration to a restoration configurationin response to the restoration indicator. An optical signal received ina first direction at a first wavelength is optically regenerated toproduce an optical signal in the first direction at a second wavelength.An optical signal received in a second direction at the secondwavelength is optically regenerated to produce an optical signal in thesecond direction at the first wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a portion of a long-haul opticalnetwork.

FIG. 2 is a schematic illustration of an optical cross-connect accordingto an embodiment of the invention.

FIG. 3 is a schematic illustration of a tunable terminal according to anembodiment of the invention.

FIG. 4 is a schematic illustration of a portion of a long-haul opticalnetwork according to an embodiment of the invention.

FIG. 5 is a schematic illustration of a portion of a long-haul opticalnetwork according to an embodiment of the invention.

FIG. 6 is a schematic illustration of a portion of a long-haul opticalnetwork according to an embodiment of the invention.

FIG. 7 is a flowchart illustrating a method according to an embodimentof the invention.

FIG. 8 is a flowchart illustrating a method according to an embodimentof the invention.

DETAILED DESCRIPTION

A method of managing changes (e.g., wavelength usage) in an opticalnetwork can include steps performed before the restoration path isneeded (also referred to as “hot standby connections”), or after therestoration path has been activated and the transients have beencompensated (also referred to as “cold standby connections”). In theformer case, restoration paths are pre-determined and are activelymaintained until used to replace a failed link elsewhere within thenetwork. For example, a restoration path can include discretebidirectional links initially maintained between nodes of a networkbefore a failure elsewhere in the network needs to be dynamicallyrestored. In such a configuration, once a failure has occurred, therestoration path can be used to restore the failed path by defining anend-to-end path among the requisite nodes. Each intermediate node can beconfigured so that eastbound signals for a given channel are received ata first wavelength and sent at a second wavelength. Similarly, westboundsignals for a given channel are received at the second wavelength andsent at the first wavelength. Following this example, the intermediatenode converts the eastbound signals from the first wavelength to thesecond wavelength during their optical regeneration, and converts thewestbound signals from the second wavelength to the first wavelengthduring their optical regeneration.

A method according to an embodiment of the invention includes receivinga restoration indicator associated with a path that includes an opticalcross-connect (OXC). The OXC is reconfigured from a standbyconfiguration to a restoration configuration in response to therestoration indicator. An optical signal received from a first directionat a first wavelength is optically regenerated to produce an opticalsignal sent in the first direction at a second wavelength. An opticalsignal received from a second direction at the second wavelength isoptically regenerated to produce an optical signal sent in the seconddirection at the first wavelength.

In another embodiment, a method includes receiving a first test signalfrom a first OXC at a second OXC. The first test signal has a firstwavelength. The second OXC has a set of wavelength-selective switches.The first test signal is passed at a first wavelength-selective switchfrom the set of wavelength-selective switches, while other signals atthe first wavelength are blocked. A second test signal is sent from thesecond OXC to the first OXC. The second test signal has the firstwavelength. Transients associated with the first test signal and thesecond test signal are compensated for to define a standby connection.In some embodiments, the first test signal is the same signal as thesecond test signal. For example, the first test signal is looped throughthe second OXC and sent back to the first OXC.

FIG. 1 is a schematic representation of a portion of an optical network10 having multiple OXCs 20, 22 and 24. An OXC as used herein can be, forexample, an integrated optical cross-connect (i.e., a generalizedoptical add-drop multiplexer that allows express wavelengths to beoptically cross-connected through an integrated OXC fabric). Anintegrated OXC can also include dense wavelength division multiplexing(DWDM) functionality. An OXC can be an optical add-drop multiplexer(OADM), which allows the signals at some wavelengths and/or formats (forexample, OC-48 or OC-192) to pass through optically, while others arerouted to terminating equipment or regenerators. An OXC can use, forexample, broadcast-and-select concepts. As described herein, an OXC isshown as the only component at a particular node within the network, butthis is for ease of illustration only. It should be understood that atany given node within a network multiple components can exist with anOXC.

OXC 20 is in communication with OXC 24 via a provisioning path 26 alonga first path within the network 10. The provisioning path 26 is thedesignated path of communication between OXC 20 and OXC 24. If a failureoccurs in the circuit, such that OXC 20 can no longer communicate withOXC 24 via provisioning path 26, a new route or path can be established.One such route or path is illustrated as restoration path 30, whichincludes a link 28 between OXC 20 and OXC 22, and a link 29 between OXC22 and OXC 24.

An example of the architecture of an OXC is shown in FIGS. 2 and 3. AnOXC 120 includes a set of splitters 131 and N×1 wavelength-selectiveswitches 132 at each port from a set of drop/add ports 133. The OXC 120also includes tunable terminals 140 each in communication with arespective drop/add port 133. Although only one tunable terminal 140 isshown in FIG. 2, each drop/add port 133 can be in communication with aseparate tunable terminal 140. The drop-side interfaces of each thedrop/add ports 133, can use, for example, standard short reach (SR)optical interfaces, such as interfaces 148, at the same frequency. Thedrop-side interfaces can be connected to routers or cross-connects, suchas the Core Director cross-connect manufactured by Ciena. Each drop/addport 133 is also in communication with an input/output fiber, such asfiber 146 shown in FIG. 2.

The tunable terminal (“terminal”) 140 is illustrated in more detail inFIG. 3. The terminal 140 includes one or more regenerators 150 (labeledas “Regen”), an N:1 coupler 149 and a 1:N splitter 151. Each regenerator150 can include a tunable laser 142, a transmitter interface board 143(labeled as “Trib TXs”), a receiver interface board 147 (labeled as“Trib RXs”), filters 144, a receiver 145, and can also include 3R(regeneration technology including three processes: re-shaping,re-timing, and re-amplification) electronics (not shown).

The splitters 131 can be configured to send copies of each signalentering the OXC 120 to each remaining add/drop port 133. The N×1wavelength-selective switches 132 can be configured to select which ofthe signals at a given wavelength to send on the output fiber 146. Thesedevices can simultaneously provide power equalization at a channellevel. The terminal 140 can add and drop signals at a given add/dropport 133. The filters 144 allow signals of any wavelength at theterminal/switch fabric interface 148 to be routed to any tributarytransmit/receive port (not shown).

FIG. 4 illustrates a portion of an optical network 210 including an OXC220 (node A) in communication with an OXC 224 (node B) via the fiberslabeled “Fiber A-B.” OXC 220 includes a set of wavelength-selectiveswitches (WSS) 232 and splitters 231 at add/drop ports 233. The OXC 220can also include one or more regenerators at each add/drop port 233,however, only a regenerator 250 (labeled “Regen A1”) is shown in FIG. 4.The regenerator(s) can be unidirectional. The OXC 220 also includes aN:1 coupler 249, a 1:N splitter 246, and filter 244. The regenerator 250includes a tunable laser 258, a Forward Error Correction (“FEC”) module262, and a photo-diode (PD) 254 configured to detect incoming signals.

The OXC 224 similarly includes a set of WSS 270 and splitters 271 atadd/drop ports 272. The OXC 224 also includes one or more regenerators,although only a regenerator 252 (labeled “Regen B1”) is shown in FIG. 4.The OXC 224 also includes a N:1 coupler 261, a 1:N splitter 262, and afilter 263. The regenerator 252 includes a tunable laser 260, a PD 256,and a FEC module 264.

Establishing a Hot-Standby Connection

In an ultra-long-haul (ULH) system, if a failure occurs along aprovisioned path, a new path can be provisioned (i.e., a restorationpath can be established). A new path is normally provisioned by tuning alaser and a receiver to a desired wavelength. If the restoration path isrelatively long, several laser/receiver pairs in sequence can be used.When a restoration path is established, non-uniform power transientsintroduced by the new wavelength can be stabilized. This stabilizationof power equalization can take many seconds, far longer than may beacceptable for all restoration situations.

A method according to an embodiment of the invention includespre-establishing a set of “hot-standby” optical connections betweenregenerators in OXCs along an alternate or restoration path within anetwork. When needed for restoration, the hot-standby opticalconnections can be dynamically connected together in series to providethe desired restoration path. Because the hot-standby connections arepre-established, the optical transients associated with activatinglasers will not be an issue at the point in time when restoration isneeded. This can allow a restoration path to be established much morerapidly.

For example, to establish a hot-standby connection between OXC 220 andOXC 224 on a wavelength λ₁, the first step is to generate a test signalat regenerator 250 (Regen A1). For example, the FEC module 262 inregenerator 250 can generate a pseudo-random bit sequence (PRBS) testsignal 266. The appropriate tunable laser, such as laser 258, tunes toλ₁ and transmits the test signal 266. The wavelength-selective switch(WSS) 232 (labeled as “A4” in FIG. 4) passes the test signal 266 to thefiber A-B, blocking other potential signals with wavelength λ₁. The testsignal 266 travels from OXC 220 to OXC 224 over the Fiber A-B. Althoughone or more optical amplifiers can exist between OXC 220 and OXC 224,such optical amplifiers are not shown for purposes of simplicity andclarity. In addition, other OXCs can exist between OXC 220 and OXC 224,which are not participating in the establishment of this connection andare therefore not included in FIG. 4. When the test signal 266 arrivesat OXC 224, the test signal 266 is broadcast by the splitter 270(labeled “Bs” in FIG. 4) to all the output ports, i.e., WSS B1, B3, andB4. WSS B1 is configured to pass signals having a wavelength λ₁ so thatthe test signal 266 is passed from splitter 270 (labeled Bs in FIG. 4)and all other potential signals at this wavelength λ₁ are blocked.

The regenerator 252 (Regen B1) then re-transmits the received testsignal 266 back to OXC 220. For example, this can be accomplished by thereceiver filter 262 located before regenerator B1 and tuned towavelength λ₁. The test signal 266 is detected by the PD 256, and theFEC module 264 of regenerator 252 (Regen B1) uses a loopback mode andre-transmits the same test signal 266 on wavelength λ₁ using the tunablelaser 260 on the regenerator 252 (Regen B1). In some embodiments, theregenerator 252 (Regen B1) can generate a second test signal to sendback to OXC 220 instead of re-sending the original test signal 266.

The regenerated test signal 266 on wavelength λ₁ is then passed throughWSS B2 and travels from OXC 224 to OXC 220 over the Fibers A-B. At OXC220, the splitter 231 (labeled as As in FIG. 4) sends the test signal266 to all output ports (i.e., WSS A1, A2, A3, and A4). WSS A1 passesthe wavelength λ₁ from splitter 231 (labeled as As in FIG. 4) and blocksall other signals. The regen 250 (Regen A1) then receives the testsignal 266.

The above-described process establishes a bi-directional connection atwavelength λ₁ between OXC 220 and OXC 224. During the hot-standby set-upprocess, the ULH network deals with any associated transients usingvarious power equalization technologies to achieve the desired qualityof service (QoS). The test signal 266 is generated at one end andloopbacked at the other end, and its individual QoS can be monitored.

Restoring a Service Connection After a Failure

FIGS. 5 and 6 illustrate a portion of another example of an opticalnetwork. A network 310 includes an OXC 320 (labeled “A”), an OXC 322(labeled “B”), and an OXC 324 (labeled “C”). The network 310 includes aworking connection for signals having a wavelength λ9 from OXC 320 toOXC 324. This connection can be a direct or provisioned path defined bya fiber pair 326. In normal operation, a transmitter 334 (TX_(A)) at OXC320 (node A) is tuned to wavelength λ9. The OXC 320 can establish aunidirectional A-to-C connection at wavelength λ9. Signals having thiswavelength are passed by a wavelength-selective switch (WSS) 332(labeled A3) and blocked by a WSS A2 and WSS A4. At OXC 324, the signalshaving wavelength λ9 are passed by a WSS 336 (labeled C1) and terminatedby a receiver (RX_(C)) 338, which is tuned to frequency λ9. The reverseconnection, from node C to node A, is done in an analogous manner. Toavoid confusion, signals in the reverse direction C-A are indicated asλ9* to distinguish them from signals in the direction A-C.

A hot-standby connection can be established between OXC 320 and OXC 322for signals having a wavelength λ1, using a regenerator 350 (labeled“A1”) at OXC 320 and a regenerator 352 (labeled “B1”) at OXC 322.Likewise, a hot-standby connection can be established between OXC 322and OXC 324 for signals having a wavelength λ2, using a regenerator 368(labeled “B2”) at OXC 322, and a regenerator 374 (labeled “C2”) at OXC324. The method of setting up these hot-standby connections can doneusing the same process as described above for the embodiment illustratedin FIG. 4.

If and when a failure occurs on the direct A-C provisioned path on fiberpair 326 that results in signals at wavelength λ9 to not be delivered,the communication between OXC 320 and OXC 324 can be restored on a path330 (see FIG. 6) that includes a link 328 between OXC 320 and OXC 322,and a link 329 between OXC 322 and OXC 324 using the hot-standbyconnections previously established.

As shown in FIG. 6, when a failure X occurs along path 326, theregenerator 350 at OXC 320 can turn its laser (not shown) off, as atransmitter 334 (TX_(A)) tunes to wavelength λ1 with the same powerlevel regenerator 350 had formerly used for its transmitter. At OXC 324,the regenerator 374 turns its laser (not shown) to off, as a transmitter376 (TX_(C)) tunes to wavelength λ2 with the same power level theregenerator 374 had formerly used for its transmitter.

At OXC 322, the regenerator 352 (labeled B1) tunes a receiver 379 fromλ1 to λ2, and the regenerator 368 (labeled B2) tunes a receiver 378 fromλ2 to λ1. Simultaneously, regenerator B1 switches from a loopback mode(e.g., test mode) to a regen mode and regenerates λ2 onto λ1. Similarly,regenerator B2 switches from loopback mode to a regen mode andregenerates λ1 onto λ2.

After this is completed, at OXC 320 a receiver 380 (RX_(A)) tunes fromwavelength λ9 to λ1, and at OXC 324 a receiver 338 (RX_(C)) tunes fromwavelength λ9 to λ2. After this, the restoration process is complete andthe restoration path 330, including link 328 between OXC 320 and OXC 322and link 329 between OXC 322 and OXC 324, is established.

FIG. 6 illustrates the wavelengths at each OXC after the restoration iscomplete. The wavelengths associated with the reverse direction C-B andB-A flows are indicated as λ2* and λ1*, respectively. Theabove-described process of establishing a restoration path on anas-needed basis when a failure occurs along the provisioned path, can becontrolled in a number of ways.

Methods of Establishing a Restoration Path

A method of establishing a hot-standby connection between a first OXCand a second OXC is illustrated in a flow chart in FIG. 7. The methodincludes at step 94, receiving a first test signal at the first OXC at afirst wavelength from the second OXC. Next, at step 96 the first testsignal is passed by a first wavelength selective switch at the firstOXC, while the first wavelength selective switch blocks other signals atthe first wavelength. A second test signal is sent from the first OXC tothe second OXC at step 98. The second test signal can be the same testsignal as the first test signal loopbacked to the second OXC. At step100, transients associated with the test signal are compensated for atboth the first OXC and the second OXC.

A method of establishing a restoration path is illustrated in aflowchart in FIG. 8. The flowchart of FIG. 8 continues from the methoddescribed in FIG. 7 (see label D at the bottom of the flowchart of FIG.7 and the top of the flowchart of FIG. 8) as is described below. Amethod includes defining one or more hot-standby connections at step 82.For example, a first OXC located between a second OXC and a third OXCalong a path within an optical network can be set-up as a hot-standbyconnection. The process of establishing the hot-standby connection wasdescribed above and was illustrated in the flowchart in FIG. 7. At step84, a restoration indicator is received at the first OXC. A restorationindicator can be, for example, a signal transmitted within a networkthat is associated with a path between the second OXC and the third OXC.The indicator notifies one or more OXCs (e.g., the first OXC) locatedbetween the second OXC and the third OXC that a restoration path isneeded. At step 86, the first OXC between the second OXC and the thirdOXC is reconfigured from a standby configuration to a restorationconfiguration as previously described. The first OXC can then receive anoptical signal in a first direction at a first wavelength from thesecond OXC and can receive an optical signal in a second direction at asecond wavelength from the third OXC at step 88. At step 90, the firstOXC can regenerate the optical signal received in the first directionand regenerate the optical signal received in the second direction. Thefirst OXC can send the regenerated signal that was received in the firstdirection at the first wavelength, in the first direction at the secondwavelength, and can send the regenerated signal received in the seconddirection at the second wavelength in the second direction at the firstwavelength.

Cold Standby Embodiment

In the establishment of a restoration path previously described, theregenerator cards used at the two ends of the restoration path,regenerator 350 (A1) and regenerator 374 (C2), are only used to generateand receive test signals for the hot-standby connections A-B and B-C.Thus, the regenerator card 350 (A1) and the regenerator card 374 (C2)are not used on the restoration path A-B-C. Instead, the originaltransmitters and receivers used by the connection between OXC 320 andOXC 324 (i.e., TX_(A), RX_(A) and TX_(C), RX_(C)), are retuned andutilized. The regenerator cards at the OXC 322, regenerator 352 (B1) andregenerator 368 (B2), are both used on the restoration path to connectthe two hot-standby connections.

In an alternative embodiment, the regenerators at each of the end nodes,OXC 320 and OXC 324, in the example shown in FIGS. 5 and 6, areeliminated since they are only used for test signals. In thisembodiment, a standby connection between OXC 320 and OXC 322, andbetween OXC 322 and OXC 324 would not be “on” prior to a failure. Thistype of standby connection is referred to as a “cold-standby”connection. As a result, a different wavelength distribution occursafter the restoration path is established. This can possibly result inslowing down the ULH convergence process after a failure, due to thenon-optimal power equalization state.

A cold-standby connection can, however, provide significant cost savingsin some cases as compared to a hot-standby connection. For example, inthe example network described above in FIGS. 5 and 6, a total of 4regenerator cards are used with the hot-standby connections (i.e.,regenerator cards A1, B1, B2 and C2). For a cold-standby connectionapplied to the OXCs illustrated in FIGS. 5 and 6, only 2 regeneratorcards are used (i.e., B1 and B2). The total savings can be significantwhen a large number of connections between the original end nodes (OXC Aand OXC C) are to be restored. For example, if there are 20 OC192connections between OXC 320 and OXC 324, and each OXC includes 20regenerator cards, a reduction of 40 regenerator cards can be realized.

Thus, an embodiment including a cold-standby connection is similar tothe above described configuration for a hot-standby connection exceptthe standby regenerator cards, A1 and C2, at the two end nodes (OXC 320and OXC 324) are eliminated. In the event of a failure in the connectionbetween OXC 320 and OXC 324, the two original end transmitter andreceiver pairs TX_(A)/RX_(A) and TX_(C)/RX_(C), can be tuned to the samefrequencies on the corresponding standby connections, λ1 and λ2,respectively, in the same manner they are retuned to such frequencies inthe hot-standby embodiment. Thus, a cold-standby connection establishesa restoration path at a slower rate than a hot-standby connection, sinceswitches tune to the designated wavelength during the establishment ofthe restoration path. In contrast, in the hot-standby connection, thedesignated wavelength has already been established and only needs to beturned on when a restoration path is needed. In a cold-standbysituation, the power equalization convergence process can be improved byusing a testing regenerator card at each end node to send and receivetesting signals periodically on all the cold-standby connections thatterminate at that node using the corresponding wavelength.

CONCLUSION

While various embodiments of the invention have been described above, itshould be understood that they have been presented by way of exampleonly, and not limitation. For example, a network can include a varietyof different components not illustrated herein. In addition, the OXCsare represented as being the only component at a particular node forillustrations purposes only. A given node within a network can includemultiple components, with the OXCs being a sub-component within a node.

1. A method, comprising: receiving a first test signal from a firstoptical cross-connect (OXC) at a second OXC, the first test signalhaving a first wavelength, the second OXC having a plurality ofwavelength-selective switches; passing the first test signal at a firstwavelength-selective switch from the plurality of wavelength-selectiveswitches while blocking other signals at the first wavelength; sending asecond test signal from the second OXC to the first OXC, the second testsignal having the first wavelength; and compensating for transientsassociated with the first test signal and the second test signal todefine a standby connection.
 2. The method of claim 1, wherein thestandby connection is a first standby connection, the method furthercomprising: receiving a third test signal from a third OXC at the secondOXC, the third test signal having a second wavelength; passing the thirdtest signal at a second wavelength-selective switch from the pluralityof wavelength-selective switches while blocking other signals at thesecond wavelength; sending a fourth test signal from the second OXC tothe third OXC, the fourth test signal having the second wavelength; andcompensating for transients associated with the third test signal andthe fourth test signal to define a second standby connection.
 3. Themethod of claim 1, further comprising: after the compensating, receivinga restoration indicator associated with a path that includes the secondOXC; reconfiguring the second OXC from a standby configuration to arestoration configuration in response to the restoration indicator; andoptically regenerating an optical signal received in a first directionat the first wavelength to produce an optical signal in the firstdirection at a second wavelength.
 4. The method of claim 1, furthercomprising: after the compensating, receiving a restoration indicatorassociated with a path that includes the second OXC; reconfiguring thesecond OXC from a standby configuration to a restoration configurationin response to the restoration indicator; optically regenerating anoptical signal received in a first direction at the first wavelength toproduce an optical signal in the first direction at a second wavelength;and sending the optically regenerated signal to a third OXC in the firstdirection at the second wavelength.
 5. The method of claim 1, furthercomprising: after the compensating, receiving a restoration indicatorassociated with a path that includes the second OXC; reconfiguring thesecond OXC from a standby configuration to a restoration configurationin response to the restoration indicator; optically regenerating anoptical signal received in a first direction at the first wavelength toproduce an optical signal in the first direction at a second wavelength;and optically regenerating an optical signal received in a seconddirection at the second wavelength to produce an optical signal in thesecond direction at the first wavelength.
 6. The method of 1, furthercomprising: after the compensating, receiving a restoration indicatorassociated with a path that includes the second OXC; reconfiguring thesecond OXC from a standby configuration to a restoration configurationin response to the restoration indicator; optically regenerating anoptical signal received in a first direction at the first wavelength toproduce an optical signal in the first direction at a second wavelength;optically regenerating an optical signal received in a second directionat the second wavelength to produce an optical signal in the seconddirection at the first wavelength; and sending the optically regeneratedsignal received in the second direction to the first OXC in the seconddirection at the first wavelength.